STEWART, J.R.*; HEULIN, B.: Ontogeny of the Extraembryonic Membranes in Oviparous Viviparous Lizards, Lacerta vivipara.

Lacerta vivipara is a reproductively bimodal species with allopatrically distributed viviparous and oviparous populations. Morphological, ecological and genetical similarities among populations indicate that oviparous and viviparous populations are closely related and diverged recently. This species provides a rare opportunity among squamate reptiles to compare the influence of reproductive mode on intraspecific variation in development of the extraembryonic membranes. We studied ontogeny of the extraembryonic membranes of an oviparous population for comparison with classical and recent studies of viviparous populations. Morphogenesis of the yolk cleft and isolated yolk mass is typical for Squamata. The yolk cleft extends across the abembryonic surface of the yolk vesicle shortly after oviposition. The allantoic vesicle fills the embryonic hemisphere of recently oviposited eggs and a vascularized chorioallantoic membrane contacts the eggshell over the surface of this hemisphere. The isolated yolk mass has regressed and the chorioallantoic membrane completely encircles the egg in near-term embryos; both of these events are characteristic of other oviparous lizards. However, one component of the ontogeny of the yolk sac has not been reported in other species. The novel feature is a secondary yolk cleft that develops as a sheet of intravitelline cells penetrates the yolk mass internal, and parallel, to the primary yolk cleft. The allantoic vesicle grows into the secondary yolk cleft as it expands around the perimeter of the egg. Although the topology of the extraembryonic membranes in near-term embryos is similar to other oviparous species, the developmental sequence producing that topology differs.